The purpose is to gain an understanding of the structures and mechanisms of action of enzymes (acetyl-CoA carboxylase, HMG-CoA reductase, and HMG-CoA synthase) that acutely control key steps in fatty acid synthesis, cholesterogenesis and ketogenesis. This information is crucial in defining the action of an effector or hormone on the catalysis by the regulatory enzyme. Once the action of an effector on the enzyme is defined, proof of this physiological role will be sought in cell systems. Two cell culture systems will be applied as models to verify the results with purified enzymes. A chick liver cell culture system and 3T3-L1 adipocytes will be employed for that purpose. Long range objectives are to determine: -the structural mechanistic basis for acute control of catalysis by acetyl-CoA carboxylase which controls fatty acid synthesis. -the mechanisms of action of HMG-CoA reductase and HMG-CoA synthase which control cholesterogenesis and ketogenesis. Specific aims are to determine: 1. The kinetics of an correlation between citrate-induced polymerization (into filaments) and activation of the protomeric form of acetyl-CoA carboxylase; the roles of physiological effectors on these processes; 2. the dependence of activity of the carboxylase on the position of the protomer-polymer equilibrium; the roles of citrate, phosphorylation state of the enzyme, etc., in control of this equilibrium; 3. The occurrence and role of the reversible transition of the protomeric form of carboxylase into active polymeric filaments in the intact chick liver cell & 3T3-L1 adipocyte; 4. the mechanism and role of the phosphorylation and dephosphorylation of carboxylase in homogeneous form and in the intact chick liver cell and 3T3-L1 adipocyte; 5. the mechanism of action of HMG-CoA reductase & HMG-CoA synthase with focus on the role of acyl-enzyme intermediates.